This invention relates generally to the fabrication of semiconductor devices; and more particularly to the fabrication of high-frequency bipolar transistor structures. The misalignment of the emitter regions with respect to the higher impurity concentration base regions normally encountered while trying to scale down geometric dimensions of active semiconductor regions is eliminated by the use of a permeation-etching process which allows for the self-aligning of the emitter regions with respect to the base regions. The permeation-etching process is described in:
a. "The Dry-Ox Process for Etching Silicon Dioxide," April 1977, Solid State Technology by Richard L. Bersin and Richard F. Reichelderfer; and in:
b. "IPC Dry Ox Etches: Selective Etching of Silicon Dioxide," Product Bulletin 7409, International Plasma Corporation. Devices having a very small separation between the emitter and higher impurity concentration base regions may be produced resulting in reduced base resistances which allow for high-frequency operation of the devices.
U.S. Pat. No. 3,489,622 issued Jan. 13, 1970 shows a transistor having higher and lower impurity concentration base regions. The higher impurity concentration base regions are formed by using impurities from a doped silicon dioxide layer by heating so as to diffuse the impurities from the doped layer into the semiconductor substrate. Openings in the doped oxide layer are used to define the emitter and lower impurity concentration base regions. In this manner, this technique, also purportedly self-aligning in nature, defines the emitter, lower impurity concentration and higher impurity concentration base regions in a single masking step. Practically, however, this technique, being dependent on diffusion rather than ion implantation, for example, for the transport of impurities into the semiconductor substrate, results in the movement of the impurity atoms in all three dimensions; such movement commencing from the surface of the semiconductor substrate and not from some depth within the substrate. This results in extending the higher concentration base regions into and overlapping with the emitter regions. This, to some extent defeats the self-aligning purpose of this technique. Also, this technique is difficult to control in a production environment.
Another technique of making bipolar structures involves the formation of the higher impurity concentration base regions and the emitter and lower impurity concentration base regions in separate masking operations. This is generally done by first defining and forming the higher impurity concentration base regions using one masking operation and then forming the lower impurity concentration base and emitter regions using a subsequent masking operation. The latter masking operation invariably results in the misalignment of the emitter region with respect to the base regions, and even more so when geometries of these regions are in the order of microns or submicrons, thereby rendering the lower impurity concentration base regions as low resistance regions or resulting in an increased base resistance via the higher impurity concentration base region.
In yet another technique, the higher impurity concentration base regions are first formed uniformly across a semiconductor substrate using ion implantation. Regions are then masked with oxide to allow for the formation of the lower impurity concentration base regions and the emitter regions. These regions are formed at a depth within the substrate greater than the higher impurity concentration base regions. The higher impurity concentration base regions above the emitter regions are appropriately compensated by the introduction of impurities of an opposite kind to those used to form the higher impurity concentration base regions. This technique results in very poor final yields as the ion-implantation step of forming the high impurity concentration base regions damages regions of the substrate wherein the emitter is to be located. Also, the frequency response of devices made using this technique is poor owing to the increased depth location of the lower impurity concentration base regions in relation to the higher impurity concentration base regions. Further, this technique, too, is difficult to control.